Cold-formed steel (CFS) built-up sections are increasingly utilized in structural applications due to their lightweight properties, high strength-to-weight ratio, and enhanced moment capacity, which can be further improved by adding stiffeners to the webs or flanges. Accurately determining the moment capacity of these sections is important for practical use. This can be achieved through experimental testing or finite element analysis (FEA) using a four-point bending setup. However, conducting full-scale experimental tests is both expensive and time-consuming, and developing a full-scale finite element (FE) model to replicate these tests can be computationally intensive. To address these challenges, this study introduces an accurate, efficient, and robust numerical method that replicates the behaviour of CFS built-up sections, applicable to both back-to-back and face-to-face configurations. The proposed numerical methodology demonstrates excellent agreement with available test results in terms of moment capacity and failure modes, highlighting its potential for determining the moment capacity of built-up sections. The study further investigates two cross-sectional types—stiffened nested and unstiffened nested—to evaluate their behaviour and moment capacity. The proposed simplified model was subsequently used in an extensive parametric study to determine the moment capacity of stiffened and unstiffened nested sections. Parameters considered in the study include varying section thicknesses (0.75 mm to 3.00 mm), web depths (150 mm to 600 mm), section configurations (nested and stiffened nested), and screw spacings (75 mm, 150 mm, and 300 mm). Given that existing design standards, such as those from the American Iron and Steel Institute (AISI 2016) and the Australian/New Zealand Standards (AS/NZS 2018), lack specific guidelines for calculating the moment capacity of these sections, this research aims to fill that gap. Furthermore, to simulate practical scenarios, models incorporating screws in the webs (screw-web model) were developed and compared against the simplified model. The results of the parametric study revealed that the moment capacity of the built-up sections is significantly influenced by section thickness, web depth, and section configuration. Screw spacing, however, had a less significant impact. Notably, the moment capacity of the screw-web model was lower than that of the simplified model, emphasizing the importance of bimoment and warping deformation in connection design. The study concludes that the developed simplified model provides a computationally efficient tool for predicting the moment capacity of built-up beams, offering valuable insights for designers and engineers. Additionally, the nested section configuration is recommended for achieving higher moment capacity compared to the stiffened nested section.